1. Field of the Invention
The present invention relates to the slotted mode in wireless cellular or personal communications services (PCS) Code Division Multiple Access (CDMA) systems wherein idle slots appear sandwiched between leading and trailing active frame portions or partial frames and the mobile station makes signal quality measurements during the slots, e.g. of pilot strength, at candidate operating frequencies different from the current or serving operating frequency. In its particular aspects, the present invention relates to adjustment of transmission parameters made with respect to communications in the leading and trailing frame portions to mitigate deterioration of communications and/or dropped calls.
2. Description of the Related Art
It is specified in EIA/TIA IS 2000.5 section 2.6.6.2.8.3.3 that the mobile station measures the strength of all pilots in a candidate frequency search set in one or more visits to the candidate frequency, using actions including stopping various activities at the serving frequency, tuning from the serving frequency to the candidate frequency, measuring mean input power at the candidate frequency, returning to the serving frequency, and restarting activities thereon. These inter-frequency measurements are transmitted to the network infrastructure where they are used in making handover decisions.
Handovers are employed in wireless cellular and personal communications systems to allow mobile stations to travel from the coverage area of one base station to another, including the ability to maintain the continuity of an ongoing call. While handovers are usually employed to transfer the mobile station from a current to a new base station, it is also possible to hand over a communication with the mobile station from one set of forward and reverse links to another of the same base station.
In a CDMA system a handover of a mobile station between base stations can be either a soft handover, in which during a transient period the mobile station simultaneously maintains forward and reverse links with both the current and new base stations, generally at the same frequency, and receives the same data via both base stations, and a hard handover, where the mobile station switches from current links to new links generally at a new frequency and/or of a new network without such a transient period in which current and new forward and reverse links are simultaneously maintained.
The known slotted mode is employed to provide idle slots during which the mobile station makes the inter-frequency measurements. The idle slots are usually, but not necessarily equal to a frame in duration, and are immediately preceded and immediately succeeded by frame portions or partial frames which are typically one half frame in duration. For ease of explanation, each combination of a leading frame portion, a slot, and a trailing frame portion is referred to herein as a xe2x80x9cslotted framexe2x80x9d, and these xe2x80x9cslotted framesxe2x80x9d are interspersed among the usual frames referred to herein as xe2x80x9cnormal framesxe2x80x9d. The instantaneous symbol rate RINFO, and instantaneous spreading factor SF=RCHIP/RINFO (where RCHIP is the constant chip rate of the Pseudo Noise (PN) spreading code) in these frame portions or half frames are maintained to be the same as in the normal frames. Then, in order to aid the recovery by the Viterbi decoder of symbols lost due to the lack of throughput during the slot, the power level, and consequently the symbol energy to noise ratio, of transmissions in these frame portions is increased accordingly.
However, it is not always possible to sufficiently further increase the power level of transmission, e.g. when the power level of the mobile station is already at or near its maximum radiated power, which leads to increases in bit error rate (BER). Under such circumstances, the slotted mode may cause a dropped call, causing annoyance and inconvenience to the parties to the call.
It is an object of the present invention to reduce the occurrence of dropped calls as a result of the slotted mode in CDMA wireless cellular and personal communication systems, by a method which is an alternative to, or can be used in addition to, increasing transmission power during the frame portions preceding and/or succeeding slots.
The present invention is based on the insight that to substantially obviate a high risk of a dropped call during or as a result of the slotted mode, an increase in BER may be mitigated by acts including sufficiently increasing the spreading factor of transmissions to and from the mobile station during these frame portions.
Thus, a slotted mode method in accordance with the present invention employed in a wireless CDMA communication system which slotted mode includes slotted frames interspersed among normal frames, each slotted frame comprising a slot during which a measurement of signal quality of transmission at a candidate operating frequency different from the serving frequency is made by the mobile station, a leading frame portion preceding the slot, and a trailing frame portion succeeding the slot, is characterized in that data in the leading and trailing active frame portions of the slotted frame is transmitted at a spreading factor which is greater than, and preferably at least double, that of data in the normal frames.
Such a increase of spreading factor may be as an alternative to or in addition to increasing transmitting power during the active frame portions of the slotted frame.
Other objects, features and advantages of the present invention will become apparent upon perusal of the following detailed description when taken in conjunction with the appended drawing, wherein: